A typical electric drive system for a mobile machine, such as a mining truck, includes a prime mover to produce a mechanical output received by a generator. For example, the prime mover may be an internal combustion engine and the mechanical output may be a rotation of a crankshaft. The generator is driven by the mechanical output to produce an alternating current (AC) output. A first set of power electronics, such as rectifiers, may receive the AC output and convert the AC to a direct current (DC) output. The DC output is then passed through a second set of power electronics, such as inverters that convert the DC output to an AC output with a desired frequency. When the mobile machine is operating in a traction mode, this AC output is then used to drive one or more traction motors, which in turn drive one or more wheels and propel the mobile machine.
The mobile machine may also be capable of doing regenerative braking (e.g., braking, decelerating, downshifting, etc.). During regenerative braking, the traction motors function as generators by converting the kinetic energy of the mobile machine to electrical energy. The energy efficiency of the mobile machine may rely on how this regenerated electrical energy is treated. One way of treating the regenerated electrical energy is to dissipate it as heat through a retarding grid of resistors. To avoid overheating, a grid blower may be used to cool the retarding grid. Meanwhile, the engine still needs to consume fuel to power all parasitic loads and/or auxiliary devices during regenerative braking or idling of the mobile machine. Thus, in this way, the regenerated electrical energy is wasted without reducing any fuel consumption of the engine.
For fuel efficiency and environmental purposes, it is desirable to use the regenerated electrical energy to at least partially power the mobile machine and its subsystems. For example, the mobile machine may store the regenerated electrical energy in energy storage devices for later use. The stored energy may be used to power the parasitic load and/or auxiliary devices so as to reduce engine involvement and fuel consumption. A drawback of this arrangement is that the usually heavy energy storage devices add extra weight to the mobile machine and may increase fuel consumption in the traction mode.
Alternatively, the regenerated electrical energy may be used by the generator working as a motor to drive the engine. However, because the rectifiers only allow power flowing in a single direction, i.e., from AC to DC, additional power conversion units are needed to reversely communicate the regenerated electrical energy to the generator. One such solution is described in U.S. Pat. No. 7,034,480 (the '480 patent) issued to Kumar et al. on Apr. 25, 2006. The '480 patent discloses a drive system that feeds the regenerated electrical energy into a main alternator to rotate an engine. When the system is operating in a traction mode, the AC output by the main alternator is converted by a rectifier to DC power. The DC power is subsequently converted by multiple traction motor inverters to three-phase AC power provided to multiple traction motors. The system further includes a switch-type contactor and an additional inverter between the main alternator and the multiple traction motor invertors. The contactor is used to control the activation of the additional inverter. The additional converter and the rectifier are connected in a parallel configuration. When the system is working in a regenerative braking mode, AC power from the multiple traction motors is converted to DC power by the multiple traction motor inverters. The system then closes the contactor and thereby activates the additional inverter to convert the DC power into AC power to rotate the main alternator, such that the engine may produce power without fuel use.
Although the drive system of the '480 patent may provide an electrical pathway for using the regenerated electrical energy to power the engine, the system may lack electrical stability. In particular, the parallel-connected additional inverter can generate fast-changing transient voltage that may turn on the diodes in the rectifier, causing erroneous current circulation. Such current circulation not only causes power loss, but also leads to current distortion disturbing other sensitive loads and equipment in the drive system. In serious cases, the current distortion may cause voltage spikes, circuit overheating, damages to control system, etc. Moreover, the transient voltage may cause high voltage buildup at the slow-responding diodes of the rectifier. This high voltage may generate electrical arc that damages the diodes and/or the contactor.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.